Small gap dynamics in the southern boreal forest of eastern Canada: Do canopy gaps influence stand development?

Abstract

Question: To what extent do small‐scale disturbances in the forest canopy, created by natural disturbance agents, affect stand development? Doubts exist as to whether small canopy openings have any real effect on the understory tree recruitment, especially in boreal forests. Location: Conifer and mixed stands in the Gaspesian region in eastern Québec. The main natural disturbance agents are recurring outbreaks of Choristoneura fumiferana (eastern spruce budworm) and winds. Methods: Linear transects in 27 sites were used to describe the gap (< 0.1 ha) regime parameters, including gap fraction, gap size and change in disturbance severity through time. Three stand types were distinguished, based on a gradient of abundance of tree host species for the eastern spruce budworm. The impact of gaps was evaluated on the basis of changes in the number, the period of recruitment, and the composition of tree saplings present within gap areas. Changes were measured along the gap size gradient, and according to the pattern of recent budworm epidemics. Results: The gap fraction is highly variable (18%‐64%) and is on average relatively high (42%). Gap sizes have a positively skewed distribution. In most cases the growth rate among gap filling saplings increased sufficiently to date disturbance events. The composition and the structure of understory trees were affected by gap formation. The number of shade‐intolerant tree species did increase during or following periods of particularly severe canopy disturbances. However, the establishment or survival of shade intolerant species was not restricted to larger gaps or more intensely disturbed periods. Conclusions: In sub‐boreal forests of Eastern Canada, small scale disturbances in the tree canopy influence stand regeneration dynamics, but not to the extent that parameters such as sapling composition and recruitment patterns depend on gap regime characteristics.

Northward migrating trees establish in treefall gaps at the northern limit of the temperate–boreal ecotone, Ontario, Canada

“…(Pinaceae)], two abundant canopy tree species of the boreal forest, also established in similar sized gaps thereby representing dominant overstory eVects in these studies. This has led Hill et al (2005) andde Römer et al (2007) to conclude that succession trajectories were unaVected by gap disturbances in the boreal forest. Additionally, red maples were absent both in gaps and the understory/canopy in each of these studies (Kneeshaw and Bergeron 1998;Hill et al 2005;de Römer et al 2007 represented over 75% of the total tree abundance and basal area in gaps of a hardwood mixed forest in New Hampshire (2°s outh of our study site), representing a dominant overstory eVect not observed at Wolf Lake.…”

“…In southern boreal forests, 2° north of our study site, paper birch, a prototypical shade-intolerant pioneer species (Frelich and Reich 1995a), established at high abundances in treefall gaps of similar sizes as those found at Wolf Lake (»50-500 m 2 ) (Kneeshaw and Bergeron 1998;Hill et al 2005;de Römer et al 2007). Black spruce and balsam Wr [Abies balsamea (L.) Mill.…”

Boreal mixedwood stand dynamics: ecological processes underlying multiple pathways

“…In the absence of a stand-initiating disturbance, forest structure and composition are regulated by gap dynamics (Frelich and Reich 1995, Kenkel et al 1997, Römer et al 2007, Kneeshaw et al 2011. In the eastern boreal forest, where a lower fire frequency allows sufficient time for stands to experience canopy disturbance, shade-tolerant species recruit into gaps and forests develop a multi-cohort structure with a mosaic canopy of conifer species such as balsam fir or eastern white cedar (Frelich and Reich 1995, McCarthy 2001, Chen and Popadiouk 2002, Pham et al 2004).…”

“…The conifer stand could then transition to a mixedwood condition and be maintained as such, displaying mixedwood gap dynamics (Kneeshaw and Bergeron 1998, Senecal et al 2004, Römer et al 2007, Reinikainen et al 2012. Spruce budworm defoliation or windthrow could be causal factors producing this dynamic Chen 2011, Reinikainen et al 2012).…”

“…evidence for all these possibilities in the east (e.g., Senecal et al 2004, Römer et al 2007, Taylor and Chen 2011, and in Manitoba (Levac 2012) and Alberta (Shongming Huang, Alberta Environment and Sustainable Resource Development, personal communication, the "dynamic alternating moving mosaic") as well.…”